In order to explain the background of the invention, reference will be particularly made to FIG. 1. The reference numeral 11 designates a pre-amplifier intended to amplify data which is read out from a memory cell accessed by an address signal from the outside. The detailed construction thereof is shown in FIG. 2. The reference numeral 12 designates a main-amplifier intended to amplify the output of the pre-amplifier 11, and output the amplified signal as an output data Do. The detailed construction thereof is shown in FIG. 3.
The reference numeral 13 designates a driver circuit intended to output a driving signal for driving the main-amplifier 12 to the node N3. In the driver circuit 13 which is constituted by internal transistors Q1, Q2, Q3 and internal nodes N1, N2, the drain of the transistor Q1 is connected to the power supply (Vcc) terminal, and the source thereof is connected to the third node N3 at which the driving signal is obtained. The drain of the transistor Q2 is connected to the third node N3, the source thereof is connected to the earth (Vss), and the gate thereof is connected to the drain of the transistor Q3. The drain of the transistor Q3 is connected to the first node N1, the source thereof is connected to the earth (Vss), and the gate thereof is connected to the second node N2.
The following description of operation is based on the use of N-channel MOS transistors as the internal transistors Q1 to Q3.
In FIG. 4 which shows a timing diagram in a so-called hidden-refresh mode: (a) shows an inverted signal of a Row Address Strobe signal (hereinafter referred to as RAS); (b) shows an inverted signal of a Column Address Strobe signal (hereinafter referred to as "CAS"); (c) shows an address signal Ai from the outside; (d) shows a signal at the internal node N1; (e) shows a signal at the internal node N2; (f) shows a driving signal at the internal node N3; (g) and (h) show the outputs D and D from the pre-amplifier 11, respectively; and (i) shows the output data Do.
Hereupon, the hidden-refresh is operated in such a manner that when the RAS signal falls down on a condition that the CAS signal is low, a refreshing of all the memory cells which have the row address designated by the row address signal is executed and completed at the time when the RAS signal rises up. On the other hand, at a time when the CAS signal falls down while the RAS signal is low, data of the memory cell which has the row and column address designated by the address signal at that time is output from the memory as the output data Do. The "hidden-refresh" means that the refreshing of the memory cells is executed while data is output from the memory.
At first the operations of the pre-amplifier 11 and the main-amplifier 12 are described with reference to FIG. 5, wherein (a) shows the RAS signal, the (b) shows the CAS signal; (c) and (d) show the signals of the I/O bus line 23 and I/O bus line 24 respectively; (e) shows a set signal .phi.s input to the pre-amplifier 11; (f) shows a signal at the node N4; (g) shows a signal .phi.c input to the pre-amplifier 11; (h) and (i) shows the outputs D and D from the pre-amplifier 11 respectively; (j) shows the driving output at the node N3; (k) and (l) show signals at the node N7 and N8 respectively; and (m) shows the data output Do of the driving circuit 13.
The information stored in the memory cell selected among the memory cells of the memory cell array 21 is transmitted to the I/O bus line 23 and I/O bus line 24 as complimentary signals (refer to FIG. 5(c), (d)) through the I/O control circuit 22. The I/O bus line 23 and the I/O bus line 24 are connected to the drains of the transistors Q5 and Q6 respectively, both of which constitute a flip-flop in the pre-amplifier 11. The flip-flop is activated by the rising of a set signal .phi.s (refer to FIG. 5(e)) input to the pre-amplifier 11 which signal turns on the transistor Q21, thereby making the node N4 discharge. The nodes N5 and N6 are pre-charged to a high voltage through the transistors Q7 and Q8 by the pre-charge signal .phi.p, respectively. Now it is supposed that the I/O bus line 23 is at "H" (high level), the I/O bus line 24 is at "L" (low level), and these signals are amplified by the flip-flop, then the transistor Q9 is turned off, the transistor Q10 is turned on, and the node N6 is discharged to the earth through the transistors Q10 and Q21. The transistor Q11, connected between the node N5 and the output D, and the transistor Q12, connected between the node N6 and the output D, are both cut-off transistors. This enhances the boot-strap effect of the main-amplifier 12.
In the main-amplifier 12, the voltage at the driving output node N3 rises at the time when the outputs D, D of the pre-amplifier 11 are transmitted to the gates of the transistors Q13, Q14, whereby the voltages of the nodes N7, N8 are determined in accordance with the outputs D, D of the pre-amplifier 11 (refer to FIG. 5(k), (l)). The voltages of the nodes N7, N8 are amplified by the transistors Q15, Q16 in flip-flop connection. The transistors Q17 and Q18 are arranged so as to discharge completely the gate voltage of either of the transistors Q13 and Q14 which is in OFF state. Either of the output transistors Q19, Q20 is turned on in accordance with the voltage of the nodes N7, N8, thereby determining the level of data output Do (refer to FIG. 5(m)).
The circuit of FIG. 1 will be operated as follows:
When the hidden-refresh operation is started, the node N2 (refer to FIG. 4(e)) becomes "H", and the transistor Q3 is turned on, making the node N1 (refer to FIG. 4(d)) "L". When the node N1 becomes "L", the transistor Q2 is turned off, thereby rising the level of driving output N3. The RAS signal (refer to FIG. 4(a)) arises making the node N2 "L", whereby the outputs D, D of the pre-amplifier 11 are reset.
The mode shown in FIG. 4 is that where the CAS signal is made "L" in the first cycle, the column addresses are taken out to execute a read-out cycle, thereby maintaining the output data Do as it is, and thereafter the RAS-only-refresh is continued with the CAS signal being held at "L" in the usual RAS-only-refresh mode. Meanwhile, the output data Do from the main-amplifier 12 is held.
At this stage, it is necessary to keep the driving output for driving the main-amplifier 12 at the node N3 (refer to FIG. 4(f)) at "H" in order to keep the output data Do (refer to FIG. 4(i)). It is necessary that the transistor Q2 of the driver circuit 13 be placed in a completely OFF state in order to keep the driving output N3 at "H".
However, the voltage of the node N2 which is connected to the gate of the transistor Q3 which is arranged so as to place the node N1 to the earth, which node (N1) is connected to the gate of the transistor Q2, will become "L" at the rising of the RAS signal shown in FIG. 4(a), as apparent from FIG. 4(e), with keeping the state "L" during the hidden-refresh mode. Accordingly, the "L" level of the node N1 (refer to FIG. 4(d)) is an unstable one which is not connected to the earth (Vss) during the hidden-refresh. As the RAS system circuit operates periodically during the hidden-refresh, the voltage of the node N1 varies to the plus side caused by the coupling of noise voltages that arise accompanying the circuit operation of the RAS system circuit, thereby decreasing the voltage of the driving output N3. The outputs D and D of the pre-amplifier 11 (refer to FIG. 4(g), (h)) are reset by the rising of the RAS signal (refer to FIG. 4(a)).
Under such construction shown in FIG. 1, the voltage of the driving output N3 decreases during the hidden-refresh operation, unfavorably affecting the holding of the level of output data Do.